1. Field of the Invention
This invention relates to an element comprising solid material deposited onto fibrous material which is secured to one surface of a support.
2. Discussion of the Prior Art
Many processes, in particular printing processes, are known in which solid material is deposited onto a support, e.g. a polymeric film, in order to modify one or more of its properties, e.g. its appearance or its electrical properties. In such processes, the solid material may be applied to the support as a solution or a dispersion in a suitable liquid medium which is subsequently removed, leaving the solid material in place. While satisfactory results can be achieved in this way, there are many combinations of supports and solid materials which are insufficiently compatible to give entirely satisfactory results, especially when the support is flexible. One important class of products which can be produced in this way are conductive elements.
Conductive elements comprising laminates of a porous electrically conductive layer composed of electrically conducting particles bonded together in an open continuous structure, electrodes on the conductive layer in electrical contact therewith and at least one layer covering each surface of the conductive layer are disclosed in U.S. Pat. No. 2,952,761 to R. Smith-Johannsen. Numerous methods for the preparation of similar elements for use as heaters are disclosed in the prior art. See, for example, U.S. Pat. Nos. 2,803,566, 2,891,228 and 2,991,257 all to R. Smith-Johannsen, 3,900,654 to H. J. Stinger, 3,400,254 to Takemori and 3,876,968 to R. D. Barnes et al. The disclosures of the above patents are incorporated by reference herein.
In the prior art processes the conductive composition is applied to a substrate, for example, asbestos fiber mat, glass fabric or thermoplastic film. The structure is then generally laminated to one or more outer layers. When the conductive composition is an aqueous based dispersion of conductive particles, as in above-mentioned U.S. Pat. No. 2,952,761, the substrate used in actual practice has generally been an asbestos fiber mat, although use of other substrates is suggested in this patent. When the conductive composition is one containing an organic solvent medium, a wider range of substrates, including thermoplastic films, can be used. For example, in U.S. Pat. No. 3,900,654, a heating element is prepared from a thermoplastic film support with an adherent conductive layer of an electrically conductive material containing carbon black dispersed in a fluorocarbon elastomer. It is mentioned that the support can be composed of a layer of the polymer adhered to another material such as a fibrous sheet. It is also mentioned that the conductive elastomer can be applied from a liquid coating composition in which the elastomer is dispersed in an organic solvent or in water. Conductive elements prepared by processes of the type described above are not entirely satisfactory. In particular, when the support is not adequately wetted by the liquid composition comprising conductive particles, so-called "mud-cracking" of the conductive layer can take place when the liquid composition is dried, resulting in unstable electrical properties, especially when the support is flexible. While a very useful product can be made by impregnating a uniform asbestos fiber mat with an aqueous dispersion of conductive particles, careful precautions are necessary in handling products including asbestos fibers, and attempts to replace the uniform asbestos fiber mat by other uniform fiber mats have not yielded satisfactory products.